1. Field of the Invention
This invention concerns mooring systems for floating production, storage and offloading (FSPO) vessels and the like, which require anchoring to the seabed. In particular, the invention concerns chain tables by which anchor legs are secured to the vessel.
2. Description of the Prior Art
Prior art arrangements for turret moored vessels include a chain table secured to the bottom end of the turret by which anchor legs are coupled between the seabed and the turret. Prior arrangements have included permanent securement between the bottom of the turret and the chain table. Other prior arrangements have provided a buoyant disconnectable chain table which sinks below the vessel on disconnection and provides for rapid disconnection of the vessel from its mooring.
FIGS. 1A and 1B of the drawings illustrate prior art arrangement for mooring a turret to the seabed with anchor legs. FIG. 1A from U.S. Pat. No. 4,606,727 is typical of some prior art chain tables for turrets in that horizontal flanges extend from the bottom of the turret for connection at 100 to anchor legs. Other turrets such as illustrated in FIG. 1B have several sockets arranged at the base of the turret with chain tubes and winches placed at an upper deck for pulling in anchor legs. The sockets are each arranged and designed to accept a fairlead or coupling assembly. The number of sockets available for the fairlead assemblies and the anchor legs is greater than the number of actual anchor legs which will be used to moor the vessel in order to provide flexibility in mooring patterns. Nevertheless, providing excessive numbers of chain pipes, sockets and fairleads is expensive in terms of cost and weight for the mooring system.
3. Identification of Objects of the Invention
A primary object of the invention is to provide an improved chain table by reducing the size, complexity and cost of the chain table described above.
Another object of the invention is to provide a simpler, less massive chain table as compared to prior art chain tables, thereby reducing the cost of a turret/chain table assembly.
Another object of the invention is to provide a lower cost chain table that can be configured to accept several arrangements of mooring lines, such that a single chain table design, for example, can be used for several turret/chain table mooring applications, each one with a different mooring line pattern.
Another object of the invention is to provide an improved chain table that is arranged to provide a reduction of size (as compared to prior chain tables) while being capable of handling expected mooring loads (for example, the same loads applied to the prior chain tables).
Another object of the invention is to provide a chain table for a turret of a turret moored system, where the reduction of size of the chain table, as compared to prior chain tables, enables fabrication by less expensive casting methods, rather than more expensive plate welding methods.
Another object of the invention is to provide a method of manufacturing a chain table by casting methods.
Another object of the invention is to provide a method for installing anchor legs of a turret by using a standardized chain table.
Another object of the invention is to provide a method for installing anchor legs of a turret moored vessel with partial connection of anchor leg portions at port and final installation of anchor legs at the mooring location.
The objects identified above, as well as other features and advantages of the invention, are provided in a cast chain table including a central ring and top and bottom outwardly extending flanges which are angled downwardly from a horizontal position of the ring that is attached to the bottom of the turret. The ring and flanges are preferably cast as one piece. A plurality of pairs of holes in the top and bottom flanges are provided to accept a shaft through a hole of a coupler assembly by which an anchor leg may be pivotally secured to the chain table. The downward angle of the top and bottom flanges aids in balancing anchor leg forces on the chain table (as compared to prior art arrangements).
According to another aspect of the invention, a method for installing anchor legs of a turret is provided by providing a standardized chain table with a plurality of bosses on a flange thereof and forming holes in certain of the bosses according to an anchor leg pattern for mooring the vessel at an offshore location.
According to another object of the invention, a method for installing anchor legs of a turret moored vessel is provided by first connecting vessel end portions of anchor legs to a chain table before the vessel arrives at the mooring site, installing a seabed portion of anchor legs according to a pre-established anchor leg pattern, and after the vessel arrives at the mooring site, connecting respective vessel end portions to seabed portions of the anchor legs.